Oil soluble friction modifiers – once called friction reducers – have been used many years by the lubricant industry. Many products made use of friction reducers:
– Automatic Transmission Fluids (ATF’s or those designed for smooth clutch engagement)
– Limited Slip Gear Oils for limited slip differentials and transaxles
– Multipurpose tractor fluids for wet brakes
– engine oils

There are also many other, lesser-known products, also containing friction modifiers in the form of animal fats, vegetable oils, sulpherized olefin coplymers, and esters.

Such products made use of friction modifiers as a way to meet performance requirements calling for smooth transitions from static to dynamic conditions and vice versa, as well as for reduced squawk, chatter, noise, frictional heat and start-up torque.

In the seventies, some gear oil additives were found to reduce frictional heat and gear operating temperatures under extreme load conditions while eliminating chatter in limited slip differentials. It was thought these same additives might be used in engine oils to accomplish the same function.

The ability to reduce friction and sometimes wear, over and above that provided by the base lubricant’s viscosity, has been called “oiliness” or “lubricity.” However, both of the latter terms are now considered obsolete. Early experimenters found that the ability of animal or vegetable fats and acids strengthened the tenacity of the oil films when incorporated in lubricating oils. These experimenters later found that the esters of vegetable or animal esters could be synthesized and produced from alcohols and acids of basic chemical compounds; what we call today as “Group V” lubricants. Their effectiveness was often rated in terms of “film strength,” an expression that still remains in use.

Much confusion has abounded in the relationship between Anti-Wear (AW) or Extreme Pressure properties, and Friction Modifiers (FM). Both friction modifiers and Anti-Wear compounds both operate in the Boundary lubrication regime. AW additives are among the type of compounds that provide good boundary lubrication. Such materials as ZDDP, sulfurized fats and esters, organometallic compounds (such as Molybdenum dithiophosphates, Molybdenum dithiocarbamates, Antimony dithiocarbamates) have shown their ability to build and maintain strong boundary lubrication films under severe load conditions and heat. However, with the exception of second-generation gear oils, the older first-generation AW additives had little FM capabilities.

The critical difference between AW/EP additive films and FM films is in their mechanical properties. AW/EP films are semiplastic deposits which are hard to shear off. Thus, under shearing conditions, their coefficient of friction is moderately to high. The exceptions are the organometallic compounds listed above. Friction modification films consist of orderly, close-packed arrays of multimolecular “whiskers,” loosely adhering to each other. The outer layers are sheared-off easily, allowing for low coefficient of friction. The phenomena can be described as a deck of plastic coated playing cards lying on the table and sliding off the top card easily.

Conversely, AW/EP films work by protecting the mating metal surfaces from asperities physically gouging the opposite surface. When a hydrodynamic film of oil is ruptured, this layer of AW/EP material protects the mating surfaces from catastrophic failure.

For some sense of scale, here are some further analogies:
1. The Coefficient of Friction (CF) of unlubricated surfaces is 0.5 and higher. In physical simulation, the process resembles the resistance of dragging an irregular rock over irregular rocky ground.
2. The CF for of friction of W/EP films is about 0.1 to 0.2. In simulation, it would resemble dragging a more or less flat stone over a flat rock.
3. The CF for a friction-modified film is about 0.01 to 0.02, compared to ice skating.
4. The CF of fully fluid films in hydrodynamic lubrication is about 0.001 to 0.006 or less. It can be compared to hydroplaning.

The preferred film is of course the hydrodynamic film. This is to followed by the friction-modified mode of operation, followed by an AW/EP regime. When high speeds or low loads are present, it is easy to maintain the hydrodynamic regime. When the speed falls, however, or the load rises above a critical point, the hydrodynamic regime breaks down and then it would be very desirable to be able to glide smoothly into a friction modification mode of operation. If no friction modification has been provided, the system defaults to a AW/EP regime. So friction modification and AW/EP is a logical method to widen the range of effectiveness of the lubricating film. Friction Modification depends much on the mechanism of contact (geometry) and molecular construction of the FM.

FM’s may be produced from a number of chemicals:
– long-chain carboxylic acids and their derivatives including salts,
– long-chain phosphoric or phosphonic acids and their derivatives
– long-chain amides, imides, and derivative
– specially prepared esters and esters of base oils.

Some of the acids used to make the salts or esters may be phenylstearic, stearic, oleic, heptanoic, benzoic, and sebacic.

The configuration of the molecule (molecular structure) of FM’s determines how many molecules are adsorbed on the surface. The slimmer molecules make stronger films because they allow closer packing. The base oil chain length also affects the strength of the adsorbed molecule. Different FM’s are required for different base oils, and the interaction of FM’s with other additives have to be investigated as well. The “concentration” of FM’s is important as well. But only so much concentration will prove effective. A concentration above a certain point may show no improvement, so cost/concentration/effectiveness has to be evaluated during tests.

FM’s can be employed in different forms in an additive package for a specific formulation. It can be added by itself without any other function, or may be part of molecule in a detergent (such as a sulfonate) or as part of a Viscosity Improver or antioxidant.

Example of an FM/Detergent additive may be a long-chain calcium, magnesium, or sodium sulfonate, preferably one long chain of the benzene ring.

Since FM’s are surface-active materials, and as such, compete with other useful additives, care must be taken in their selection and concentration in any fully formulated lubricant.